Class SieveParserVisitorImpl defines the behaviour for each visited node in
the Sieve grammar. Each method corresponds to a node type and is invoked when
a node of that type is evaluated.

In essence, this class translates between the nodes operated on by the JavaCC
generated classes and the Sieve classes operated upon by the Commands, Tests
and Comparators. A visit to the start node, ASTstart, triggers evaluation of
all of its descendants.

See https://javacc.dev.java.net/doc/JJTree.html for indepth information about
Visitor support.

Note that this class is not thread safe. It's use should be
restricted to a single thread for the duration of a visit.